Friday, April 26, 2013

Moeraki Boulders, Otago, New Zealand

These boulders, located on the southeast shoreline of New Zealand's south island, are concretions composed of calcite. Think of a concretion like a geological rubber band ball. It is composed of sedimentary rock with certain minerals acting as cement between the layers of sediment. Concretions are known for the spherical shape in which they form.

The Moeraki boulders, measuring up to 2m in width, are composed of calcite, and their "veins" consist of rare, late-stage quartz and dolomite. They were coated in a mud rock layer from the Paleocene era, a geological epoch that ended approximately 50 million years ago. However, the boulders pictured have been exhumed from their mudstone enclosure. The mineral rich cracks, or septarian veins, described earlier, have been developed over a period of several million years.

The origin of these boulders is predicted to be similar to that of an oyster pearl. On the sea floor, layer upon layer of material gradually covered a central core, such as a fossil shell or piece of wood. Aqueous minerals accumulated, and eventually concretions were generated through the process of lithification.

Today, the Moeraki Boulders are protected in a scientific reserve. Local legends believe that the boulders are vegetables that washed ashore from a canoe that wrecked on the coast of New Zealand several hundred years ago. Scientists continue to study these fascinating geological formations today, continuously learning more about their formation.

Image Credit: James McDonald


Journal of Sedimentary Research:

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